1. Field of the Invention
This invention is used in a telephone system which has a central office with a switching system and an office battery. Specifically the invention is a substation with amplifiers connected to a long line. The substation must be connected to a long telephone line to operate. Thus, the long line must be included in the invention.
2. Description of the Prior Art
In the past there have been a number of inventions of this type. Many early inventions have amplifiers in the receive branch of an ordinary telephone to increase the level. The gain in the receive branch is low so as not to make the telephone set sing. Most of the early inventions did not use precision hybrids. Many use switches to keep the substation from singing due to feedback from the transmitter to the receiver. One or more use local batteries or local A.C. power.
In more recent years there have been a great many inventions that make use of transmit and receive electronic switching devices. A signal level detector is used in each branch. The branch with the highest level is switched on while the other branch is switched off. In such an arrangement a precision hybrid is not required because the reduced gain is obtained by a switching device. Although this kind of invention may look attractive, it has the objection of the switching action which cuts one part off wholely or partly while the other is talking. It is an unnatural condition.
The new invention is an improved telephone specifically designed for a long telephone line. A precision hybrid is used to obtain a high degree of trans-hybrid loss. A precision network is used to balance the hybrid. The amplifiers are operational amplifiers which consume little power. The microphone is a telephone transmitter, and the speaker is a telephone receiver. In the past little attention has been directed to increasing the line length to make use of the amplification added. This invention is designed to increase the line length considerably.